Due to their inadequate damping elastic structures, such as for example thin metal sheets used for vehicle bodies or machine casings, emit airborne sound of different frequencies if excited by airborne sound or by structure-borne vibrations.
Hitherto, this mainly low frequency noise, especially in the range 100 to 1000 cps has been deadened by applying damping materials. Suitable materials for this purpose are viscoelastic damping foils based on bitumen and/or filled synthetic resins, as well as bituminous felts with and without additional damping coverings. The bitumen foils which are at present mainly used in the manufacture of vehicles and which are placed on the floor inside of the vehicle must have a high weight per unit area in order to bring about an effective vibration damping. Generally, the weight is approximately 4 to 7 kg/m.sup.2. However, this results only in a sound loss factor of approximately 0.1 to 0.2. l In addition, such high weights are particularly disadvantageous in vehicle building.
Materials which can be applied by spraying are also known. These are the known coatings for underbody protection of motor vehicles having a synthetic resin and/or bitumen base and which solidify to give resilient coatings of low or high bending resistance. However, these materials are mainly intended to provide good corrosion protection and high abrasion resistance. Their vibration and sound damping properties are so poor that they are inadequate without the use of the abovementioned foils inside the vehicle. Thus, conventional underbody protection materials based on filled PVC plastisols provide only a loss factor of approximately 0.02 at ambient temperature and 200 cps at a coating weight of 3 kg/m.sup.2.
It is known that sound insulation can be improved if a sandwich-like covering is formed on the sound radiating and transmitting substrate, for example a metal sheet, in such a way that a layer of resilient material, e.g. a foam material is applied to the substrate, followed by the applying thereon a layer of a material with high bending resistance and high specific gravity. Such structures are for example known from German Auslegeschrift No. 2,064,445 and although they provide considerable improvements with regard to sound insulation, they are not suitable for vibration damping and sound absorption.
U.S. Pat. No. 3,833,404 discloses vibration damping and sound-abosorbing structures formed from two layers of which the inner layer comprises a viscoelastic mixture of elastomeric and thermoplastic polymers with a modulus of elasticity of below 1.times.10.sup.10 dynes/cm.sup.2, while the outer layer comprises a rigid plastic material with a modulus of elasticity of above 1.times.10.sup.10 dynes/cm.sup.2. Due to the high rigidity of the outer layer, which may be obtained by adding reinforcing fibres, the structure thus formed is similar to a conventional sandwich system in which a viscoelastic layer is positioned between two rigid materials such as metal, wood or the like.
It is the object of the present invention to provide a process of producing sound and vibration damping coatings in which process conventional materials are applied in a simple manner, i.e. more particularly by spraying, and which process yields coatings fulfilling all requirements relative to corrosion and abrasion protection and simultaneously providing good damping agent structure-borne vibrations and good sound absorption at relatively low weights per unit area.